What is the big deal with the pokemons nature?

Every nature has like an addition to your stats: here all of them

Lonely: Attack raised, Defense compromised
Brave: Attack raised, Speed compromised
Adamant: Attack raised, Special Attack compromised
Naughty: Attack raised, Special Defense compromised
Bold: Defense raised, Attack compromised
Relaxed: Defense raised, Speed compromised
Impish: Defense raised, Special Attack compromised
Lax: Defense raised, Special Defense compromised
Timid: Speed raised, Attack compromised
Hasty: Speed raised, Defense compromised
Jolly: Speed raised, Special Attack compromised
Na�ve: Speed raised, Special Defense compromised
Modest: Special Attack raised, Attack compromised
Mild: Special Attack raised, Defense compromised
Quiet: Special Attack raised, Speed compromised
Rash: Special Attack raised, Special Defense compromised
Calm: Special Defense raised, Attack compromised
Gentle: Special Defense raised, Defense compromised
Sassy: Special Defense raised, Speed compromised
Careful: Special Defense raised, Special Attack compromised
Bashful: No change
Quirky: No change
Serious: No change
Docile: No change
Hardy: No change

The big deal is the growth of their stats.
Ex. Meganium Nat.Modest=hi Sp.A low Atk
Lv.40
Sp.A=139
Atk=87
Nat.Adamant=opp. Of Modest nature
(same level)
Sp.A=103
Atk=111

That is the best way I can explain it.

The big deal with Pokemon Nature is that it will increase certain stats of your pokemon.